Algae And Fungi Relationship – Lichens are composed of two closely related organisms, a fungus, and one or more photobiont partners. The photobiont may be an alga or a cyanobacterium, both of which are capable of photosynthesis and can generate simple sugars. By contrast, fungi are ‘heterotrophic,’ requiring nourishment from an external source. The fungus constructs the lichen thallus structure, inside which they maintain a long-term, stable connection with their photobionts, which is the foundation of the lichen symbiosis.
There is considerable disagreement regarding the precise nature of the lichen-fungi-photobionts symbiotic relationship. Are the fungi ‘farming’ the photobionts in regulated parasitism, or do the photobionts profit as well? There is compelling evidence that the lichen symbiosis is a mutualism in which both partners benefit.
While it was evident that fungi get their carbon from simple sugars, photobionts seem to be supplied with optimum living circumstances, with their numbers often being considerably more significant than those of outside lichens. Additionally, the photobiont presumably benefits from the increased availability of mineral minerals supplied by fungal digestion occurring outside its cells. Finally, but certainly not least, the inside of lichens is often densely packed, believed to aid in protection from UV radiation, desiccation, and herbivore feeding.
However, there are compelling reasons for the managed parasitism side as well. Up to half of the carbon fixed by algae is instantly transformed into fungal sugars that the algae cannot use. Second, when cultivated in the laboratory, certain lichens that establish ongoing relationships with their ‘normal’ host algae form parasitic-type interactions with non-host algae.
Indeed, it is believed that many early phases of lichen spore development may persist via a parasitic or saprophytic approach. Finally, many lichen fungus lineages are parasitic on other lichens — these are known as lichenicolous lichens! Non-lichen fungi have developed from lichenized ones in certain instances. These are often specialized opportunistic parasites, saprophytes, or even symbionts that compete for resources with other fungi in the lichen thallus.
The symbiosis may be more complicated than this. Spribille et al. have discovered yeasts buried in the cortex of ascomycete macrolichens, and their abundance corresponds with hitherto unexplained phenotypic differences. Additionally, there is compelling evidence supporting the constant presence of non-photosynthetic bacteria inside the thalli of all lichens, but their function is unclear at the moment. Interestingly, a process for non-photosynthetic bacteria was hypothesized for many years since bacteria promoted the relichenization of isolated fungus and algae in the lab.